1. Field of the Invention
The present invention relates to an electromechanical transducer such as a capacitive transducer array, and to a method of manufacturing the electromechanical transducer.
2. Description of the Related Art
Conventionally, micromechanical members to be manufactured using micromachining technology may be processed on the order of micrometers, and various functional microelements are materialized using such micromechanical members. A capacitive transducer using such technology (CMUT: capacitive micromachined ultrasonic transducer) is being studied as an alternative to a piezoelectric element. With such a CMUT, ultrasound may be transmitted and received using vibrations of a diaphragm, and in particular, excellent wide-band characteristics in a liquid can be obtained with ease.
With regard to the above-mentioned technology, there is a capacitive transducer array in which a silicon nitride film is formed on a substrate and used as a diaphragm and one of an electrode on the diaphragm and an electrode formed on the substrate is routed to a lower surface of the substrate (Japanese Patent No. 3,924,466). Further, there is a capacitive transducer array which uses a monocrystalline silicon diaphragm formed on a silicon substrate by bonding or the like (US 2008/0048211). In the configuration of the latter, a silicon film including the monocrystalline silicon diaphragm functions as a common electrode. The silicon substrate is divided and the divided silicon substrate is used as a signal take-out electrode, to thereby form the capacitive transducer array. Further, in order to improve the device stiffness, a frame structure is formed around the signal take-out electrode.
In the capacitive transducer array in which the monocrystalline silicon diaphragm is formed on the silicon substrate by bonding or the like, the silicon substrate may be divided and used as a signal take-out electrode. However, since the silicon substrate is divided, the stiffness of the transducer array is lowered and the transducer array may be broken by thermal stress in mounting or the like.
Further, an electrical signal from the electrode of the diaphragm may be taken out to a rear surface of the silicon substrate via through wiring. However, in forming the through wiring, an insulating material or a through wiring material for taking out an electrical signal to the rear surface may deposit on the diaphragm. Further, when thermally oxidizing a through hole, the monocrystalline silicon diaphragm may also be oxidized. This may cause thickness fluctuations in the diaphragm or generate stress thereon. Further, when polysilicon is used as a conductor for the wiring, the polysilicon may also deposit on the monocrystalline silicon diaphragm, which may result in thickness fluctuations in the diaphragm and stress on the diaphragm. Such thickness fluctuations in the diaphragm and stress thereon causes spring constant fluctuations and deformation fluctuations in the diaphragm. It may lead to lower consistency among elements in which transducer cells are included, and performance fluctuations among the elements in the capacitive transducer array may be increased.